GB2578763A

GB2578763A - Power umbilicals for subsea deployment

Info

Publication number: GB2578763A
Application number: GB1818146.1A
Authority: GB
Inventors: Morten Hesjevik Sven
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2018-11-07
Filing date: 2018-11-07
Publication date: 2020-05-27
Anticipated expiration: 2038-11-07
Also published as: GB2578763B; GB201818146D0; WO2020096458A1

Abstract

An integrated power umbilical 2 for subsea deployment comprises one or more power cords 4 for supplying AC power and super duplex stainless steel fluid-carrying tubing 10 adjacent to said power cord(s) 4. A zinc coating 12 covers an outer surface of said tubing 10 so as to promote corrosion resistance. The zinc coating may be applied by a thermal spraying process. The zinc coating may be between 200µm and 1mm in thickness, and preferably between 200µm and 300µm. A steel armour outer layer 6 may be provided together with a plastic sheet layer 8.

Description

Power Umbilicals for Subsea Deployment

Technical field

The invention relates to power umbilicals for subsea deployment, and in particular power umbilicals having improved corrosion protection.

Background

Corrosion in steel components immersed in water is a well-known problem.

Traditionally cathodic protection has been used to prevent corrosion. This is achieved by electrically coupling the steel component to a sacrificial piece of metal (e.g. aluminium) having a lower electrochemical potential. The sacrificial piece of metal supplies electrons to the steel component, which prevents it from corroding.

It can be difficult to achieve adequate cathodic protection for large or extended systems such as integrated power umbilicals. An integrated power umbilical is designed to serve a specific piece of equipment, such as an oil well, and to supply it with power and other services such as telecommunication and fluids. Furthermore, the umbilical consists of power cables as well as tubing containing various types of fluids. The tubing is often made from corrosion resistant alloys, such as Super Duplex Stainless Steel (SDSS). However, at elevated temperatures (e.g. above 20 °C) there is a risk of pitting and crevice corrosion on SDSS. The reason for this increased risk of corrosion is related to the formation of a biofilm on the surface of the steel. This biofilm causes a catalytic effect on the cathode reaction, leading to more noble electrochemical potential on the steel which increases the risk of crevice and pitting corrosion. It is therefore common to extrude a polymeric sheet over the tubing to provide a coating that prevents water from accessing the underlying steel.

Statement of invention

Aspects of the present invention provide integrated power umbilicals for subsea deployment, and methods of manufacturing such.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief description of figures

Figure 1 is a schematic diagram of a cross section of an integrated power umbilical according to an embodiment; and Figure 2 is a flow diagram illustrating the steps of a method of manufacturing an integrated power umbilical according to an embodiment.

Description of preferred embodiments

A power umbilical provides electric power to consumers, often located subsea. Electric power consumers can be pumps, turbines or other equipment. An integrated power umbilical having an SDSS tubing covered by a polymeric sheet can provide good corrosion protection. However, during handling, for example when deploying the umbilical, the polymeric sheet may be damaged in small areas (e.g. scuffs and bending cracks). Such coating damage is not a problem for most systems, but can be surprisingly detrimental to an integrated power umbilical supplying AC power under water. Recent experiments have shown that coating damage can lead to AC corrosion in an integrated power umbilical. The polymeric sheet, being an insulating material, causes a build-up of induced AC voltage in the steel tubing. The resulting current in an area where the coating is damaged causes high AC corrosion rates. Without the polymeric sheet, build-up of induced AC voltage does not occur, but then pitting and crevice corrosion becomes a problem caused by the elevated temperatures from the power cable.

Figure 1 shows a schematic diagram of an embodiment of an integrated power umbilical 2, which addresses at least some of the problems outlined above. The umbilical 2 comprises three power cords 4 for supplying AC power to a subsea component (not shown). The power cords 4 are enclosed by an outer steel armour layer 6. The outer layer 6 is covered by an outer plastic sheet 8. Also enclosed by the outer layer 6 is metal tubing 10 (three tubes shown), preferably comprising a corrosion resistant metal such as SDSS. The tubing 10 is covered by a zinc coating 12 (e.g. 200 pm to 300 pm in thickness), which provides an unbroken continuous cover over the tubing 10. In Figure 1, the umbilical 2 has been deployed and sea water 14 surrounds the power cords 4 and the tubing 10 in the voids.

The zinc coating 12 may prevent water from accessing the tubing 10, like a polymeric sheet would have, and thereby prevent corrosion in the tubing 10. However, unlike a polymeric sheet, because of the electric conductive properties of zinc, there is no buildup of induced AC voltage in the tubing 10. Hence, even if the zinc coating 12 is damaged, this does not cause high AC corrosion rates in the damaged area of the tubing 10.

The zinc coating 12, having a lower noble electrochemical potential than the tubing 10, also provides cathodic protection. More importantly, zinc acts as a weak biocide in water 14 and will prevent formation of a biofilm on the surface of the tubing 10 at elevated temperatures. Exposed to sea water, the zinc will corrode and it is not expected that the zinc coating 12 will be intact on the tubing surface throughout the design life of the umbilical 2. However, release of zinc ions into the confined water phase (i.e. the sea water that penetrates into the voids of the umbilical when immersed) will prevent the formation of a biofilm and hinder potential ennoblement which could lead to pitting and crevice corrosion. Although not shown, the umbilical 2 would normally also comprise further conduits, for example, to supply water or other fluids.

Preferably, the zinc coating 12 is applied in a thermal spraying process to achieve an adequate (unbroken) coverage and a desired thickness. The coating may have a thickness in the range of 200 pm to 1 mrn, or preferably a thickness in the range of 200 pm to 300 pm. A thicker coating 8 may provide a longer lifetime, but if the coating 8 is too thick, then bending cracks are more likely to occur. This may not be an important consideration for a fixed and rigid system, but may be important for an integrated power umbilical, which has some mechanical flexibility.

Figure 2 shows a flow diagram which illustrates the steps of a method of manufacturing an integrated power umbilical according to an embodiment. The method comprises: providing one or more power cords for supplying AC power (step Si), providing a metal tubing adjacent to said power cord(s) (step S2), and applying a zinc coating to an outer surface of said tubing (step S3). The step (53) of applying the zinc coating may comprise thermally spraying zinc onto the tubing. Preferably, the thermal spray is used to apply a zinc coating with a submillimetre thickness, e.g. in the range of 200 pm to 300 km. The illustrated method may be used to manufacture an integrated power umbilical as shown in Figure 1. In an alternative embodiment, the metal tubing is provided before the one or more power cords.

In summary, embodiments of the invention can enable the use of subsea integrated power umbilicals without the risk of AC corrosion, whilst also avoiding pitting corrosion and crevice corrosion at elevated temperatures.

Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.